This invention relates to thermal ink jet printheads for use in an ink jet printer, and more particularly to such printheads having ink flow directing valves to reduce back flow caused by vaporized ink bubbles used to expel ink droplets from printhead nozzles.
In existing thermal ink jet printing, the printhead comprises one or more ink filled channels, such as disclosed in U.S. Pat. No. 4,463,359 to Ayata et al., communicating with a relatively small ink supply chamber at one end and having an opening at the opposite end, referred to as a nozzle. A thermal energy generator, usually a resistor, is located in the channels near the nozzles a predetermined distance therefrom. The resistors are individually addressed with a current pulse to momentarily vaporize the ink and form a bubble which expels an ink droplet. As the bubble grows, the ink bulges from the nozzle and is contained by the surface tension of the ink as a meniscus. As the bubble begins to collapse, the ink still in the channel between the nozzle and bubble starts to move towards the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in the separation of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity of the droplet in a substantially straight line direction towards a recording medium, such as paper.
One problem with this thermal ink jet process is that the bubble growth is symmetrical, thus forcing as much ink towards the supply reservoir in the printhead as is driven out of the channels through the nozzles in the forms of droplets. The droplet velocity of the ejected droplets could be increased, if the pressure force produced by the bubbles were preferentially directed towards the printhead nozzles. This control of the bubble force directions would reduce the required power, improve droplet directionality, decrease the printhead heating during operation, and thus improve printhead energy efficiency.
U.S. Pat. No. 5,072,241 to Shibaike et al. discloses a roofshooter type ink jet printhead having a shutter which either aligns an aperture with the printhead nozzles or covers the printhead nozzles. A series of electrodes on opposite sides of the shutter cause the shutter to move and be electrostatically held in one of the two desired locations.
U.S. Pat. No. 4,774,530 to Hawkins discloses a thermal ink jet printhead which comprises an upper and a lower substrate that are mated and bonded together with a thick film insulative layer sandwiched therebetween. One surface of the upper substrate has etched therein one or more grooves and a recess which, when mated with the lower substrate, will serve as capillary-filled ink channels and ink supplying manifold, respectively. The grooves are open at one end and closed at the other end. The open ends will serve as the nozzles. The manifold recess is adjacent the groove closed ends. Each channel has a heating element located upstream of the nozzle. The heating elements are selectively addressable by input signals representing digitized data signals to produce ink vapor bubbles. The growth and collapse of the bubbles expel ink droplets from the nozzles and propel them to a recording medium. Recesses patterned in the thick film layer expose the heating elements to the ink, thus placing each of them in a pit, and provide a flow path for the ink from the manifold to the channels through an elongated trench, thereby enabling the ink to flow around the closed ends of the channels. The trench in the thick film layer eliminates the fabrication steps required to open the groove closed ends to the manifold recess, so that the printed fabrication process is simplified.